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Drought. The word evokes images of barren fields, dying stock,
and water holes and reservoirs drying to cracked mud. Shrivelled
hopes, failed crops, and often economic ruin are its
trademarks.

Drought is also part and parcel of life in Australia,
particularly in the marginal areas away from the better-watered
coasts and ranges. Of all the climatic phenomena to afflict
Australia, drought is probably the most economically costly: major
droughts such as that of 1982/83 can have a major impact on the
national economy. Moreover, apart from crop failure and stock
losses, droughts set the scene for other disastrous phenomena, such
as fires, dust-storms, and general land degradation.

Why is Australia drought prone?

Australia is prone to drought because of its geography. Our
continent sits more or less astride the latitudes of the
subtropical high pressure belt, an area of sinking, dry, stable air
and usually clear skies. The far north and south of the country
come under the influence of reasonably regular rain-bearing
disturbances for at least part of the year, and the east coast is
watered reasonably well by moisture from the Tasman and Coral Seas.
However over most of the country rainfall is not only low, but
highly erratic.

Many, but by no means all, droughts over eastern and northern
Australia accompany the El Niño-Southern Oscillation phenomenon,
which typically lasts about a year, as in 1982/83. Droughts in the
western areas and over much of the interior normally have different
causes. Nevertheless, on some occasions (such as 1914 and 1994) El
Niño-related droughts may extend across virtually the entire
country. On such occasions, the economic and livestock losses are
exacerbated

Long-term droughts

Over much of the country, droughts can extend over several years,
relieved only by brief, transitory rains. Indeed, probably the most
damaging type of drought is when one or two very dry years follow
several years of generally below-average rainfall. The "Federation
drought" of the late 1890s through 1902 is an example, as is the
more recent 1991-95 drought in Queensland, northern New South Wales
and parts of central Australia. Over still longer time-scales,
Australia's rainfall history features several periods of a decade
or longer that seem to have been distinctly "drought prone". For
instance, the mid to late 1920s and the 1930s were a period of
generally low rainfall over most of the country, continuing through
most of the 1940s over the eastern states. A similar dry spell
occurred in the 1960s over central and eastern Australia. During
these low rainfall periods, not every year is dry; it is just that
rainfall in most years is below the long-term average, and there
are often runs of years with recurrent drought. Thus in the late
1930s-40s major droughts occurred over eastern Australia in
1937-38, 1940-41, and 1943-45.
The 1990s saw formal Government acknowledgement that drought is
part of the natural variability of the Australian climate, with
drought relief for farmers and agricultural communities being
restricted to times of so-called "exceptional circumstances". In
other words, the agricultural sector was expected to cope with the
occasional drought, and relief would be available only for droughts
of unusual length or severity.

Heat and cold

Australia is not only a dry country, but is also subject to fierce
heat. As the sun tracks into the southern hemisphere in early
spring, it begins to strongly heat northern and interior parts of
the country. By November, average maximum temperatures have already
climbed to the high 30s over wide areas of northern and central
Australia - and into the low 40s in parts of northwestern Australia
and western Queensland. The heat does not relent until the
following autumn.

Heat waves

On occasions when the synoptic situation is favourable, the hot
air extends south over the southern States. In South Australia and
the southeast, this occurs when high pressure systems lie to the
east, and a cold front is advancing from the west - a combination
that directs a hot northerly airstream across these states. In
southwestern Australia, the hottest conditions are normally
associated with low pressure troughs that direct east to
northeasterly winds from the hot interior. In both cases, eastward
movement of the front or trough introduces cooler air from the
oceans or higher latitudes - the so-called, and often eagerly
awaited, "cool change".

These "hot weather" patterns occasionally become slow-moving, and
the trough or front bringing the cool change may stall, or even
dissipate. On such occasions, very high temperatures - high 30s or
even 40s - can persist for days, and in inland areas, for weeks on
end. These are the "heatwaves" of the southern States. Such
heatwaves can lead to heat exhaustion, and even death, particularly
among the very young or old. Heatwaves have, in fact, accounted for
more deaths in Australia than any other natural hazard: according
to Emergency Management Australia the January 1939 heatwave in
South Australia, Victoria and NSW killed 438
people.

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The sideways movement of air in the lower atmosphere due to the
differences in air pressure (commonly called
wind). Process of transfer of air mass properties
by the velocity field of the atmosphere.

Anomaly

The departure of an element from its long-period average value
for the location concerned. For example, if the maximum temperature
for June in Melbourne was 1 degree Celsius higher than the
long-term average for this month, the anomaly would be +1 degrees
Celsius. The current international standard is to use the 30 year
average from 1961 to 1990 as the long-term average.

Anticyclones

Atmospheric circulations that rotate anti-clockwise in the
southern hemisphere. Anticyclones are areas of higher pressure and
are generally associated with lighter winds and fine and settled
conditions.

Australian Height Datum (AHD)

An average of the level of the sea around Australia in between
high and low tide. It is used as the zero point when measuring or
marking water height (measured straight up) e.g. on a map or flood
marker. As an example, thirty metres above this 'sea level' would
be written as '30m(AHD)'.

Barometer

A tool to measure air pressure which lets us predict change in
weather.

Beaufort wind scale

A scale that uses observations of the effects of wind to
estimate its speed.

Blizzard

Violent and very cold wind which is loaded with snow, some of
which has been raised from snow covered ground.

Carbon dioxide

A gas (CO2) present in the atmosphere which plays an
important role in the greenhouse effect.

Celsius temperature scale

Thermodynamic scale of temperature. Temperature in degrees
Celsius can be obtained from value in degrees Fahrenheit by the
following formula: C = (F - 32) x 5/9

Chance of any rain

Chance of rain describes the likelihood of receiving a
measurable amount of rain (>0.2mm) during the day at that
location. For example, if the chance of rain for Mildura is 30%, it
means that on 3 out of 10 days with similar weather conditions
rainfall will be measured in the Mildura rain gauge. Where there
may be a 30% chance of any rainfall, there is also a 70% chance of
not receiving any rainfall at all.

Chlorofluorocarbons

Chemicals that release chlorine atoms that destroy ozone high in
the atmosphere.

Cirrus cloud

High cloud, delicate, hair-like and feathery looking.

Climate

The atmospheric conditions for a long period of time, and
generally refers to the normal or mean course of the weather.
Includes the future expectation of long term weather, in the order
of weeks, months or years ahead.

Cloud

A group of water drops or ice crystals that can be seen in the
sky, caused by water vapour rising and cooling. Rain, hail or snow
falls when the cloud can hold no more water or ice.

Cloud cover

Forecasting terms:

Clear: Free from cloud, fog, mist or dust haze.

Sunny: Little chance of the sun being hidden by cloud.

Cloudy: More cloud than clear sky. For example, during the day
the sun would be hidden by cloud for a lot of time.

Overcast: Sky completely covered with cloud.

Combined Sea and Swell

Also known as total wave height, or significant wave height.
Combined sea and swell describes the combined height of the sea and
the swell that mariners experience on open waters. The height of
the Combined sea and swell refers to the average wave height of the
highest one third of the waves.

Convection

The movement of warm air, rising off the land, which helps to
make cloud, local breezes, wind and thunderstorms.

Cumulonimbus cloud

Heavy, puffy, heaped, dark very tall clouds often bringing rain.
Some have an anvil shaped head. Sometimes called a 'thunder
head'.

Cumulus

Clouds with a woolly, heaped appearance that often produce
rain.

Cyclogenesis

The rapid development of a low or intensification of a
pre-existing one.

Cyclone

In Australia, ­ a cyclone is a large clockwise loop of low
pressure air that usually brings very strong or damaging winds,
unsettled weather, cloudiness and very heavy rainfall.

(NOTE: Anticyclones - In Australia, an anticyclone is a large
anti-clockwise loop of high pressure air, generally with light
winds and fine and settled weather.)

Cirrus cloud

High cloud, delicate, hair-like and feathery looking, that may
be seen on 'fine' days.

Desert

There are many different definitions which describe a desert. In
general, arid areas (or deserts) are areas with low
precipitation.

Dew

Droplets of water deposited when air cools and the water vapor
in it condenses.

Dew-point temperature

This is a measure of the moisture content of the air and is the
temperature to which air must be cooled in order for dew to form.
The dew-point is generally derived theoretically from dry and
wet-bulb temperatures, with a correction for the site's elevation.
If the dry-bulb temperature is the same as the dew-point, the air
is said to be saturated and the relative humidity is 100%.

Downburst

Violent and damaging downwards flow of air hitting the surface
of the Earth violently, usually in a severe thunderstorm.

Drizzle

Steady(rain) in very small water droplets (less than 0.5 mm in
diameter) very close to one another.

Drought

Drought is a long, unusually dry period when there is not enough
water for normal needs.

Dry-bulb temperature

This is the shade temperature (degrees Celsius) registered by a
mercury-in-glass thermometer exposed in a white louvered box or
meteorological screen which is raised on legs one metre above the
ground.

Dry

(As used by forecasters) Free from rain. Normally used when
preceding weather has also been relatively dry, and dry weather is
expected to continue for a day or so.

Dust storm

A wind storm which carries large amounts of dust or sand high
into the air and may drop them far away e.g. dust from farm areas
falling on a city.

East Coast Lows

East Coast Lows are intense low-pressure systems which happen on
average several times each year off the eastern coast of Australia,
in particular southern Queensland, NSW and eastern Victoria. They
generally last only a few days, but can become worse over night.
They can bring very strong winds, very heavy rainfall and sometimes
cause coastal flooding.

El Niño

Nowadays, the term El Niño refers to the extensive warming of
the central and eastern Pacific that leads to a major shift in
weather patterns across the Pacific. In Australia (particularly
eastern Australia), El Niño events are associated with an increased
probability of drier conditions.

ENSO

Stands for El Niño-Southern Oscillation. 'El Niño' used here
refers to the warming of the oceans in the equatorial eastern and
central Pacific; Southern Oscillation is the changes in atmospheric
pressure (and climate systems) associated with this warming (hence
'Southern
Oscillation Index'' to measure these changes). 'ENSO' is used
colloquially to describe the whole suite of changes associated with
an 'El Niño' event - to rainfall, oceans, atmospheric pressure
etc.

Equatorial trough

Zone of relatively low pressure which lies between the
subtropical anticyclones of the two hemispheres.

Fahrenheit temperature scale

Thermodynamic scale of temperature. Temperature in degrees
Fahrenheit can be obtained from value in degrees Celsius by the
following formula: F = (9C/5) + 32

Fine

Dry weather with no rain of any kind, hail or snow.

Flash Flood

Flood of short duration with a relatively high peak
discharge.

Flood

A flood occurs when water inundates (covers) land which is
normally dry.

Flood Forecasting

Scientific opinion, based on past records and on weather and
water conditions, about the likely height, amount of water, and
time a flood will probably happen, and go on for, at a particular
place on a river, stream or over land.

Flood Warning

Official advance notice that a flood may happen soon at a
certain place or near a certain river.

Fog

A dense mass of small water droplets or particles in the lower
atmosphere.

Front

The boundary between air masses having different
characteristics.

Front (Cold)

In some regions along the polar front, cold dense air advances
equatorwards, causing warm air to be forced aloft over its sloping
surface. This portion of the polar front is known as a cold
front.
Cold polar air is replacing warm tropical air.

Front (Warm)

In other regions along the front, warm air of lower density
moves polewards, sliding over its sloping surface. This portion is
called a warm front. Warm tropical air replaces cold polar air.

Frost

Deposit of soft white ice crystals or frozen dew drops on
objects near the ground; formed when surface temperature falls
below freezing point.

Gale Warning

A Gale Warning is a statement which warns of sea winds
averaging from 34 knots and up to 47knots (equal to about 60 - 85
kph) in coastal waters and out at sea.

Greenhouse effect

A natural warming process of the earth. When the sun's energy
reaches the earth some of it is reflected back to space and the
rest is absorbed. The absorbed energy warms the earth's surface
which then emits heat energy back toward space as longwave
radiation. This outgoing longwave radiation is partially trapped by
greenhouse gases such as carbon dioxide, methane and water vapor
which then radiate the energy in all directions, warming the
earth's surface and atmosphere. Without these greenhouse gases the
earth's average surface temperature would be about 33 degrees
Celsius cooler.

Global radiation

Global (short wave) radiation includes both that radiation
energy reaching the ground directly from the sun, and that received
indirectly from the sky, scattered downwards by clouds, dust
particles etc.

Gust

A gust is a sudden burst of stronger wind that usually only
lasts a few seconds.

Hail

Small roundish pieces of ice ('hailstones') (generally between 5
and 50 millimetres across,) which fall from clouds as separate bits
or in clumps.

Harden Up

'Harden Up - Protecting Queensland' is a Green Cross Australia
project. Harden Up is a statement of our ability as humans to
endure natural disasters and be resilient to our changing climate.
The term comes from a special report from the Australian Strategic
Policy Institute. The report,
called Hardening Australia: Climate change and national
disaster resilience, encourages Australians to
build resilience in infrastructure, business and the home and to be
prepared for the unique risks that Australians will face as our
climate changes.

Heat wave

A period of abnormally hot weather lasting several days.

High pressure

Atmospheric circulations that rotate anti-clockwise in the
Southern Hemisphere. Anticyclones are areas of higher pressure and
are generally associated with lighter winds and fine and settled
conditions.

Humid

Moisture in warm air.

Humidity

Moisture in warm air.

Hurricane Force Wind Warning

A Hurricane Force Wind Warning is a statement which warns of
winds averaging 64 knots or more in coastal waters and high seas
areas.

Hydrology

An earth science concerned with the occurrence, distribution and
circulation of waters on and under the earth's surface, both in
time and space, their biological, chemical and physical properties,
their reaction with the environment, including their relation to
living beings.

Hydrometeorology

The study of the atmospheric processes that affect the water
resources of the earth, including the study of the atmospheric and
land phases of the hydrological cycle with emphasis on the
interrelationships involved.

Indigenous Weather

Indigenous Australians have long held their own seasonal
calendars based on the local sequence of natural events.

Inshore

The waters within 12 sea miles (about 22kms) of the coast.

Inversion, temperature

A temperature inversion occurs when the temperature of air
increases with increasing height. Generally the temperature
decreases with height in the lower atmosphere, called the
troposphere. Low-level inversions generally form on clear calm
nights due to cooling of the ground through loss of heat by
radiation. The warm air on the ground is replaced by cooler air at
the surface resulting in a temperature inversion. The inversion
creates a boundary layer that restricts vertical motion and mixing
of air between the two air masses either side. Low-level inversions
act like a lid to trap pollutants resulting in smog over our
cities.

Isobars

Lines on weather maps running through places which have the same
air pressure.

Jet stream

A powerful current of air high above the Earth.

Knot

Measure of wind speed equal to about 1.8 km per hour.

Køppen's classification of climates

Classification of climate based on annual and monthly means of
temperature and precipitation (rainfall) which also takes into
account the vegetation limits. It is a tool for presenting the
world pattern of climate and for identifying important deviations
from this pattern.

King tide

On Australia's east coast, the highest tides happen during the
winter months of June, July and August, and the summer months of
December, January and February. The two highest tides, one in
winter and one in summer, are known as the 'king tides'.

Land Breeze

A local offshore wind. At night, when the land cools more
quickly, and to a greater extent, than the sea, the land breeze
circulation is set up. Cooler air from the land flows offshore to
replace the warm air rising over the sea. The air in contact with
the sea warms and expands and the resulting changes in the pressure
and temperature differences and distributions cause the land breeze
circulation.

La Niña

The extensive cooling of the central and eastern Pacific Ocean.
In Australia (particularly eastern Australia), La Niña events are
associated with increased probability of wetter conditions.

Lightning

The flash of light from a large electrical spark from or inside
a cloud. Lightning can injure people and animals and damage trees
and buildings if it reaches the ground.

Low latitudes

The southern hemisphere low-latitudes are considered to be the
areas of the Earth north of about 30 degrees latitude. For
Australia, this means the area north of a line from halfway between
Perth and Geraldton (in Western Australia) to Bourke (in New South
Wales). This part of Australia generally experiences a subtropical
to tropical climate.

Low pressure

Atmospheric circulations that rotate clockwise in the southern
hemisphere. Cyclones are areas of lower pressure and generally
associated with stronger winds, unsettled conditions, cloudiness
and rainfall.

Mid latitudes

The areas between about 30 degrees and 55 degrees latitude. For
Australia, this is the area south of a line from halfway between
Geraldton and Perth (in Western Australia) to Bourke (in New South
Wales). This part of Australia generally experiences a temperate
climate.

Mist

Similar to fog, but visibility remains more than a
kilometre.

MJO

Stands for Madden-Julian Oscillation, also known as the 30-50
day wave. This is a periodic enhancement of rainfall over the
Australian tropics, which progresses across tropical latitudes
roughly every 30-50 days. Satellite cloud loops and atmospheric
pressure changes can signal passage of the wave over Australia,
signalling a burst in monsoon (rainfall) activity during the
tropical wet season.

Monsoon

A seasonal wind. The northern Australian monsoon season
generally lasts from December to March. It brings cloud and heavy
rainfall over northern Australia. The north Australian wet season,
October to April, includes the monsoon months but can last
for several months on either side. Parts of the North Queensland
coast also have quite heavy rainfall throughout the cooler months.

MSL

MSL stands for Mean Sea Level. It is necessary to convert the
pressure readings to equivalent mean sea level pressures, otherwise
the important horizontal changes in pressure would be overwhelmed
by vertical variations simply due to differences in height between
observing stations. In this way, a Mean Sea Level Pressure (MSLP)
map will then show pressures affected by changing weather
conditions, not because of changing altitude.

Nitrogen

The most abundant gas in air, comprising 78% by volume. It is
colourless and odourless.

Offshore

The coastal waters zone between 12 and 60 'nautical' miles
(about 22 -110 km) from the coast.

Oxygen

The second most abundant gas in air, comprising 21% by volume.
It is a colourless and odourless gas.

Ozone

One of the several gases that make up the Earth's atmosphere. It
is the triatomic* form of oxygen and makes up approximately one
part in three million of all of the gases in the atmosphere. If all
the ozone contained in the atmosphere from the ground level up to a
height of 60 km could be assembled at the earth's surface, it would
comprise a layer of gas only about 3 millimeters thick, and weigh
some 3000 million tonnes. Ozone is toxic at high concentrations
because it reacts strongly with other molecules.

* Each ozone molecule is made up of three oxygen
atoms.

Precipitation

Any or all of the forms of water particles, whether liquid (e.g.
rain, drizzle) or solid (e.g. hail, snow), that fall from a cloud
or group of clouds and reach the ground. (See Drizzle, Rain).

Duration of precipitation

Brief: Short duration.

Intermittent: Precipitation which ceases at
times.

Occasional: Precipitation which while not
frequent, is recurrent.

Frequent: Showers occurring regularly and
often.

Continuous: Precipitation which does not
cease, or ceases only briefly.

Periods of rain: Rain is expected to fall most
of the time, but there will be breaks.

Isolated: Showers which are well separated in
space during a given period.

Local: Restricted to relatively small
areas.

Patchy: Occurring irregularly over an
area.

Scattered: Irregularly distributed over an
area. Showers which while not widespread, can occur anywhere in an
area. Implies a slightly greater incidence than isolated.

Sporadic: scattered or dispersed in respect of
locality or local distribution. Charaterised by occasional or
isolated occurrence.

Widespread: Occurring extensively throughout
an area.

Probable Maximum Flood (PMF)

The most severe flood that scientists think is likely to happen
at a particular place. The 'worst-case scenario' flood that would
be caused if all the worst weather and water conditions happened at
once.

Probable Maximum Precipitation (PMP)

The most rain that scientists think is likely to fall in a
certain time over a certain size storm area at a particular place
at a certain time of year.

Probabilities, or Probabilistic Forecasts

These forecasts use percentages, such as 60% (60 percent or 60
chances in every 100). This kind of forecast is usually based on
how often something has happened in the past. For instance,
if the chance of getting more than average rainfall if certain
things happen is said to be 60%, then in 60 out of 100 of past
years when things were similar it was higher than usual but in 40
out of 100 it was below.

High percentages do not mean something will happen, but that
scientists think it is it is likely or probable.

Rain

Precipitation of liquid usually in drops bigger than 0.5
millimetres(mm) falling more steadily than showers.

Rain day

A rain day occurs when a daily rainfall of at least 0.2 mm is
recorded.

Rainfall

The total which has fallen, as measured in a rain gauge.

Rainfall amount

Rainfall amount is the likely amount of rain in millimetres(mm)
for the forecast period. Sometimes rain falls in a patchy pattern
across an area with some places getting a heavy shower while an
area nearby might miss out completely. On these days the rainfall
range may be quite large, e.g. 5 to 30 mm. When steady rainfall is
expected over a wide area, the range may be smaller, e.g. 10 to
15mm.

Relative humidity

Is a traditional indicator of the air's moisture content. It is
the ratio of the amount of moisture actually in the air to the
maximum amount of moisture which the air could hold at the same
temperature. Relative humidity is normally expressed as a
percentage and at saturation the relative humidity will be very
close to 100%. The air can hold more moisture at higher
temperatures, hence the relative humidity alone does not give an
absolute measure of moisture content.

Ridge

A ridge is an elongated area of high pressure. It is indicated
by rounded isobars extending outwards from an anticyclone and has
associated with it a ridge line. The pressure at a point on the
ridge is higher than at an adjacent point on either side of the
line.

Sea and Swell, Combined

Combined sea and swell describes the total height of the sea and
the rise in water of non-breaking waves ('swell') out at sea.

Sea and Swell

Peak Wave Period: Period in seconds between
the swells of the primary swell component. The larger the time
difference, the greater the amount of energy associated with the
swells.

Primary Swell: Height and direction of the
swell with the highest energy component. This is sometimes referred
to as the dominant swell.

Sea waves: waves generated by the wind blowing
at the time, and in the recent past, in the area of
observation.

Secondary Swell: Height and direction of the
swell with the second highest energy component.

Swell Period: See Peak Wave Period.

Swell waves: waves which have travelled into
the area of observation after having been generated by previous
winds in other areas. These waves may travel thousands of
kilometres from their origin before dying away. There may be swell
present even if the wind is calm and there are no 'sea' waves.

Wave period: the average time interval between
passages of successive crests (or troughs) of waves.

Wave Height: Generally taken as the height
difference between the wave crest and the preceding trough.

A local onshore wind. Cooler air from over the sea flows onto
the shore to replace the warm air rising over the land. On sunny
days the land heats up more quickly, and to a greater extent, than
the sea. The air in contact with the land warms and expands and the
resulting changes in the pressure and temperature differences and
distributions cause the sea breeze circulation. At night, when the
land cools more quickly, and to a greater extent, than the sea, the
reverse land breeze circulation is set up.

Showers

Showers are often short lasting (but may last half an hour) but
can be heavy. They usually begin and end suddenly.

Sleet

Generally refers to a mixture of rain and snow or falling snow
that is melting into rain.

Smog

Smog (contraction for 'smoke fog') is a fog in which smoke or
other forms of atmospheric pollutant have an important part in
causing the fog to thicken, and have unpleasant and dangerous
physiological effects.

Snow

Precipitation of ice crystals, most of which are branched
(sometimes star shaped).

Southern Oscillation Index (SOI)

The Southern Oscillation Index (SOI) is calculated from the
monthly or seasonal fluctuations in the air pressure difference
between Tahiti and Darwin.

Squall

A squall comprises a rather sudden increase of the mean wind
speed which lasts for several minutes at least before the mean wind
returns to near its previous value. A squall may include many
gusts.

Storm Force Wind Warning

A Storm Force Wind Warning is a statement which warns of winds
averaging from 48 knots and up to 63 knots in coastal waters and
high seas areas.

Stratosphere

Layer of the atmosphere between about 10 and 50 kilometres above
the ground.

Stratus cloud

Low cloud forming a uniform layer.

A Strong Wind Warning

A Strong Wind Warning is a statement which warns of winds
averaging from 26 knots and up to 33 knots in coastal waters.

Chart showing lines of equal pressure (isobars), corrected to
mean sea level (MSL), over a broad area (eg Australia). Based on
the synoptic observations taken simultaneously every 3 hours by
weather observers and Automatic Weather Stations across
Australia.

Synoptic scale

A horizontal length scale that corresponds to the size of the
large-scale features of the lower atmosphere (ie the highs and lows
over mid-latitude regions).

Stratus cloud

Low cloud in a layer or blanket. Stratus clouds may bring very
light rain or snow. A cloudy day usually has a sky filled with
stratus clouds hiding the sun.

Sea mile, nautical mile

A measure at sea equal to about 1.8 kilometres.

Severe weather warning

Severe Weather Warnings are issued for:

Winds of gale force (blowing at 63 km/h) or more

Wind gusts (short bursts of wind) of 90 km/h or more

Very heavy rain that may lead to flash flooding

Abnormally high tides (or storm tides) expected to be higher
than usual high tide levels

Unusually large surf waves expected to cause dangerous
conditions on the coast

Southern Oscillation Index (SOI)

The Southern Oscillation Index (SOI) is a measure of changes in
the difference in measured air pressure between Tahiti and Darwin.
Because of good records about how this has related to our rainfall
in past seasons, we can use it to tell what Queensland's likely
rainfall will be in the coming months.

Strong wind warning

A Strong Wind Warning is a statement which warns of winds
averaging from 26 knots and up to 33 knots (or about 45 to 60
kph) in coastal waters. It is usually given by local or state
government agencies.

Storm surge

A storm surge is a rise above the normal water level along a
shore. In tropical areas, storm surges are caused by tropical
cyclones as they come ashore. In a storm surge the water level
rises above normal high tide levels, causing flooding on shore. The
height of the water and the speed it rises depend on the strength
of the cyclone and shape of the waterfront, among other things.

Temperature inversion

A temperature inversion occurs when the temperature of air
increases with increasing height. Generally the temperature
decreases with height in the lower atmosphere, called the
troposphere. Low-level inversions generally form on clear calm
nights due to cooling of the ground through loss of heat by
radiation. The warm air on the ground is replaced by cooler air at
the surface resulting in a temperature inversion. The inversion
creates a boundary layer that restricts vertical motion and mixing
of air between the two air masses either side. Low-level inversions
act like a lid to trap pollutants resulting in smog over our
cities.

Thunderstorm

A storm with sudden electrical discharges which make a flash of
light (lightning) and a loud rumbling sound (thunder).
Thunderstorms usually, but not always, bring rain. They usually
only last a short time and hit a small area.

Tornado

A whirlwind or mass of circling air with high wind speeds at its
centre.

Trade winds

East to southeasterly winds (in the southern hemisphere) which
affect tropical and subtropical regions, including the northern
areas of Australia. During the monsoon season in northern
Australia, the easterly trade winds are replaced by moist
northwesterly (monsoonal) winds from the Indian Ocean and southern
Asian ocean waters. As mentioned above, the trade winds in the
southern hemisphere are east to southeasterly in direction. In the
northern hemisphere however, the trade winds are east to
northeasterly in direction. It means that in both hemispheres, they
tend to blow from the east to the west and towards the equator.
Sometimes the trade winds will just be called "easterly" to avoid
having to specify the hemisphere.

Tropical cyclones

A tropical cyclone is an intense low pressure system which forms
over warm ocean waters in Queensland, The Northern Territory and
Western Australia.

A tropical cyclone usually brings strong winds and extremely
heavy rain. It can also bring unusually high sea levels (storm
surges) in areas near the coast.

A tropical cyclone can cause widespread damage as a result of
the strong wind, flooding (caused by either heavy rainfall or ocean
storm surges).It can cause landslides in hilly areas as a result of
heavy rainfall and saturated soil.

If there are winds above 117 km/h (63 knots) it will be
called a severe tropical cyclone.

(NOTE: A tropical cyclone may also be known (in other parts of
the world) as a tropical storm. In the north western Pacific severe
tropical cyclones are known as 'typhoons' and in the northeast
Pacific and Atlantic/Caribbean they are called 'hurricanes'.)

Tropical storm

Term used in the northern hemisphere for a tropical cyclone.

Trough

A trough of low pressure is an elongated area where atmospheric
pressure is low relative to its immediate surroundings. A trough of
low pressure is sometimes indicated on the synoptic chart by a
centre line or trough line denoted by a dashed line e.g. - - - - -.
The trough line often extends outward from a low pressure centre,
or an enclosed area of relatively low pressure. When moving across
a trough from one side of a trough line to another, atmospheric
pressure decreases as you approach the trough line. The atmospheric
pressure increases again after you cross the trough line and move
away. A change in wind direction will generally be observed as you
cross from one side of the trough to the other. Compare with the
definition for a Ridge and a Low.

Typhoon

Term used in the northwestern Pacific for a tropical cyclone
with maximum winds above 117 km/h (63 knots).

The name Tsunami is used to describe a series of waves
travelling across the ocean. These waves can measure up to hundreds
of kilometres between wave crests in the deep ocean. They are very
fast moving, and unlike normal waves, affect the movement of the
water all the way to the sea floor. They tend to get higher as they
get closer to land and, if high enough when they reach the land,
can travel far inland and cause very serious damage.

Earthquakes are one of the things that cause tsunami. They
can also be caused by underwater landslides, volcanoes, land
slipping into the ocean, meteors, or even the weather when the
atmospheric pressure changes very rapidly. The most common cause of
tsunami is an under sea earthquake with a sudden change in a
section of the earth's crust under or near the ocean. This rise or
fall in sea level makes a tsunami wave.

Ultraviolet radiation

Electromagnetic radiation with wavelengths shorter than visible
light, but longer than x-rays. Exposure to too much UV radiation
can cause skin cancer.

Updraught

Upward moving current of air of small dimensions. A rapidly
moving vertical wind as found in thunderstorms.

Virga

Precipitation that evaporates before it reaches the ground.

Vortex

Rotating mass of air or water, such as water going down a plug
hole.

Water Resources

Water available, or capable of being made available, for use in
sufficient quantity and quality at a location and over a period of
time appropriate for an identifiable demand.

Water Resources Assessment

Determination of the sources, extent, dependability and quality
of water resources for their utilisation and control.

Water vapour pressure

The atmospheric pressure which is exerted by water vapour (water
in its gaseous state). It is one way of measuring the humidity of
the air. At a given temperature, an increase of water vapour in the
air corresponds to an increase in the humidity of the air. Water
vapour is supplied to the atmosphere by evaporation of water from
oceans, lakes, wet land surfaces or from vegetation
(transpiration). Water vapour absorbs the Sun's radiation. As a
result, the sunlight received at the Earth's surface will be more
intense in a drier atmosphere.

Wet bulb temperature

Wet-bulb temperature is measured using a standard
mercury-in-glass thermometer, with the thermometer bulb wrapped in
muslin, which is kept wet. The evaporation of water from the
thermometer has a cooling effect, so the temperature indicated by
the wet bulb thermometer is less than the temperature indicated by
a dry-bulb (normal, unmodified) thermometer. The rate of
evaporation from the wet-bulb thermometer depends on the humidity
of the air - evaporation is slower when the air is already full of
water vapour For this reason, the difference in the temperatures
indicated by the two thermometers gives a measure of atmospheric
humidity.

Wind

Usually only one wind speed is given in a weather forecast,
unless it is expected to be very gusty. For instance, fresh, gusty
southwest winds means that wind speed will generally be between 17
and 21 knots (or about 30 to 38 kph) and the mean wind
direction will be from the southwest, but that there will also be
much stronger gusts.

Gust: a gust is any sudden increase of wind usually for only a
few seconds.

Squall: A sudden strong wind that rises up and may blow hard a
number of times before it dies down.

Surface Wind: wind speed and direction measured at 10 metres
above the earth's surface. The surface wind is what causes waves on
the ocean. Large swells can be caused by strong winds in intense
storms.

Zonal flow

1. Component of atmospheric circulation along a line of
latitude, towards the east or west.

2. Atmospheric circulation along, or approximately along,
parallels of latitude.